Voltage-activated Ca channels serve two critical functions: the regulation of cellular excitability and the regulation of Ca entry. Alterations in the density, or function, of L-type Ca channels are implicated in a variety of cardiovascular diseases. Thus, elaborating the basic mechanisms that regulate Ca channels is important for understanding both fundamental channel physiology and for therapeutic intervention. During the past funding period, we have identified the Rem GTPase as a novel modulator of I(Ca). It was originally thought that Rem association with CaVbeta- subunits chronically regulated I(Ca) by inhibiting channel trafficking. Our studies have disproved this hypothesis, demonstrating Rem-mediated Ca channel regulation without changes in surface density. Instead, Rem seems to modulate Ca channel activity through interactions with both CaVbeta and the proximal CaV1.2 C-terminus near the CB/IQ domain. Two of the most physiologically relevant controls of I(Ca) are PKA-modulation and calmodulin (CaM)-modulation. Our most recent studies suggest that Rem modulates I(Ca) responses to each of these signaling pathways. Thus, Rem appears to contribute to both beta-adrenergic and Ca-CaM control of I(Ca). The specific hypothesis to be tested is that Rem GTPase regulates Ca channel activity in cardiac muscle through interactions with both CaVb-subunits and the CaV1.2 C- terminus. Three hypothesis driven aims focus our studies and advance knowledge of this novel regulatory mechanism. Aim 1 will explore the nature of Rem-mediated channel regulation by examining the effect of Rem loss on Ca channel regulation. Initial characterization of Rem knockout mice indicates that i) Rem functions in vivo to regulate I(Ca); ii) contributes to the cardiac response to pressure-overload; and iii) contributes to cardiac myocyte growth/maturation homeostasis. Aim 2 will determine whether interaction of Rem with CaVbeta or CB/IQ is critical for modulation of I(Ca). Aim 3 will investigate how Ca- calmodulin modulates Rem-mediated channel blockade, and determine whether PKA phosphorylation alters Rem membrane trafficking or the interaction between Rem and its binding partners. RGK G-proteins function as modulators of Ca channel activity, contributing to regulation of I(Ca), excitation- contraction coupling, and the cardiac response to pressure-overload. The goal of this research is to generate a deeper understanding of the physiological ramifications and molecular mechanism of RGK/Ca channel modulation, to speed progress toward the therapeutic exploitation of RGKs in cardiovascular disease.